PETpla.net Insider 05 / 2021

BOTTLE MAKING 22 PETplanet Insider Vol. 22 No. 05/21 www.petpla.net this pressure to too low a value might allow movement of the preform bubble under the impact of high-pressure blow air and move the gate! Some machines offer a system to circulate high-pressure air through the blow mould further reducing cycle time. In either case blowing finishes around 300 degrees (Fig. 7.9) . Figure 7.9 High-pressure blow moves the preform the rest of the way to the walls of the blow mould. Mould opening The stretch rod moves out of the blow mould starting as early as 120 degrees venting is initiated with the end of blow. To facilitate this aspect, a venting valve allows air to escape via a large diameter hose or pipe ending in a suitable silencer to keep noise pollution in the plant down to a minimum. Venting finishes at 342 degrees and only then can the base be lowered. Lowering the base earlier would lead to the deformation of the bottle base known as rocker bottom. However, during venting, the clamp unlocks and at 320 degrees starts to open. This is possible because at that time pressure in the blow mould has significantly dropped. Remaining pres- sure may slightly bulge the bottle side walls outward but this will not lead to a permanent deformation. At 360 degrees the mould is open, the base insert is down and a new cycle begins (Fig. 7.10) . Figure 7.10 Pressure curve inside the blow mould here shown with optional air recov- ery. Diagram courtesy of Krones AG. 7.3 Air valve control At this point it may be beneficial to understand the functionality of the air valves that are so crucial to the proper timing of the blowing process. Today, most air valves used in blow mould- ing come in blocks of three or four: preblow, blow, exhaust, and a second exhaust valve or one used for air recy- cling. These valves are electrically triggered pneumatic slave valves as shown in Fig. 7.11. Figure 7.11 Air to and from the blow cavity is a two-step process whereby an electrically operated valve switches an air-operated one. The valve on the left is electrically operated. When it switches, it opens the path of the air through the valve on the right. These valve blocks are mounted as close to the cavities as mechanically possible to minimise “dead air” loss. Modern valves con- tain pistons made from PET or other plastic that is durable and light to allow switching in milliseconds. This is one of the “secrets” of the faster machines! Here are the different stages of valve engagement through- out the cycle. P1 stands for preblow, P2 for high-pressure blow, and EX for exhaust (Figs. 7.12–7.18). Figure 7.12 Air step 1. At the begin- ning of the cycle P1 and P2 are closed and EX is still open from the previous exhaust cycle. Figure 7.13 Air step 2. The first event is the closing of the exhaust valve. Figure 7.14 Air step 3. Now the P1 opens and the preblow phase begins. Figure 7.15 Air step 4. To initiate high- pressure blow, P2 opens at the same time as P1 closes. To prevent high-pres- sure air from getting into the preblow circuit, a check valve is mounted into the preblow line. Figure 7.16 Air step 5. The high-pres- sure valve is fully open and the con- tainer is blown to its finish dimension.